In recent years, as magnetic recording devices, such as magnetic disk drives, flexible disk drives and magnetic tape drives, have immensely expanded their ranges of utility and gained in significance, efforts have been directed toward enabling the magnetic recording media used in these devices to be prominently improved in recording density. Particularly, the increase in surface recording density has been further growing in ardency since the introduction of the Magneto Resistive (MR) head and the Partial Response Maximum Likelihood (PRML) technique. Owing to the further introduction of the Giant-Magneto Resistive (GMR) head and the Tunneling Magnet Resistive (TMR) head in recent years, the increase is continuing at a pace of about 100% per year. These magnetic recording media are being urged to attain a still higher recording density in future and their magnetic recording layers to accomplish addition to coercive force, Signal-to-Noise Ratio (SNR) and resolution. Recent years have been witnessing efforts that are being continued with the object of enhancing the linear recording density and adding to the surface recording density by increasing the track density as well.
In the latest magnetic recording devices, the track density has reached 110 kTPI. As the track density is further increased, it tends to entail such problems as causing interference between the parts of information magnetically recorded in adjacent tracks and inducing the magnetization transition region in the borderline region to constitute a noise source and impair the SNR. This fact hinders the enhancement of the recording density because it immediately results in lowering the bit error rate.
For the sake of increasing the surface recording density, it is necessary that the individual recording bits on the magnetic recording medium be formed in as minute a size as possible and enabled to secure as large saturated magnetization and magnetic film thickness as permissible. As the recording bits further decrease in size, however, they tend to entail such problems as lessening the minimum volume of magnetization per bit and inducing extinction of recorded data through the magnetization reversal caused by thermal fluctuation.
Further, since the track pitch grows small, the magnetic recording device necessitates a track servo technique of extremely high accuracy and, at the same time, generally needs adoption of the method of executing the recording in a large width and executing the reproduction in a smaller width than during the recording with a view to eliminating the influence from the adjacent tracks to the fullest possible extent. Notwithstanding that this method is capable of suppressing the influence between the adjacent tracks to a minimum, it entails such problems as rendering sufficient acquisition of the output of reproduction difficult and consequently incurring difficulty in securing a sufficient SNR.
As one means to cope with the problem of thermal fluctuation and accomplish acquisition of a due SNR or a sufficient output, an attempt to enhance the track density by forming irregularities along the tracks on the surface of the recording medium and consequently physically separating mutually the adjacent tracks is now under way. This technique will be referred to as a “discrete track technique” and the magnetic recording media that are produced by this technique will be referred to as “discrete track media” herein below.
As one example of the discrete track medium, a magnetic recording medium that is formed on a nonmagnetic substrate bestowed on the surface thereof with irregular patterns and enabled to acquire physically separated magnetic recording track and servo signal pattern has been known (refer, for example, to JP-A 2004-164692).
This magnetic recording medium has a ferromagnetic layer formed on the surface of a substrate possessing a plurality of irregularities on the surface thereof via a soft magnetic layer and has a protecting film formed on the surface of the ferromagnetic layer. This magnetic recording medium has formed in the convexed regions thereof magnetic recording regions magnetically divided from the environments.
According to this magnetic recording medium, it is held that a high-density magnetic recording medium issuing no great noise can be formed because the fact that the occurrence of magnetic walls in a soft magnetic layer can be suppressed results in preventing the influence of thermal fluctuation from readily appearing and allowing extinction of interference between the adjacent signals.
The discrete track technique is known in two kinds, i.e. a method which forms a track subsequent to forming a magnetic recording medium consisting of a number of stacked thin films and a method which forms a thin-film magnetic recording medium either directly on the surface of a substrate or subsequent to forming irregular patterns on a thin-film layer ready for the formation of a track (refer, for example, to JP-A 2004-178793 and JP-A 2004-178794). The former method, often called a magnetic layer processing-type method, is at a disadvantage in suffering the medium to be readily contaminated during the course of production and greatly complicating the process of production as well because it requires the physical processing of surfaces to be carried out subsequent to the formation of the medium. The latter method, often called an emboss processing-type method, though not inducing ready contamination during the course of production, is at a disadvantage in disabling stabilization of the posture and the height of floatation of the recording and reproducing head adapted to execute recording and reproducing while floating on the medium because the irregular shape formed on the substrate is fated to continue existence on the film to be formed.
A method for forming regions intervening between magnetic tracks of a discrete track medium by injecting nitrogen ions or oxygen ions or radiating a laser into a preformed magnetic layer has been disclosed (refer to JP-A HEI 5-205257). The regions between the magnetic tracks that are formed by this method, however, suffer survival of insufficiently magnetized state because of high coercive force in spite of low saturated magnetization and incur the trouble of inducing blurred portions in the data written in the magnetic tracks.
Further, in the manufacture of the so-called patterned medium having magnetic recording patterns disposed with definite regularity per bit, the formation of magnetic recording patterns by etching due to radiation of ions or by imparting amorphousness to the magnetic layer has been disclosed (refer to Technical Report of IEICE, MR2005-55 (2006-2), pp. 21-26 (The Institute of Electronics, Information and Communication Engineers) and U.S. Pat. No. 6,331,364). This method, however, has entailed the problem that the magnetic recording medium incurs contamination and the surface thereof suffers decline of flatness during the course of manufacture and the problem that the irradiation with ions brings about no sufficient demagnetization of the magnetic layer and the irradiation with ions inflicts damage on the magnetic layer and induces decline of flatness of the surface thereof.
This invention, in a magnetic recording medium encountering technical difficulty in consequence of addition to recording density, is directed to eliminating the blurring of the data during the magnetic recording and consequently adding to the areal recording density by markedly increasing the recording density and suppressing the coercive force in the regions between the magnetic recording pattern portions and the residual magnetization to the fullest possible extent while retaining the recording and reproducing properties at least above the conventional level. Particularly with respect to the discrete track-type magnetic recording medium that imparts the undulation subsequent to the deposition of the magnetic layer on the substrate, this invention contemplates providing a method of manufacture that decisively simplifies the procedure of manufacture by excluding the step of demagnetizing the magnetic layer and precluding occurrence of noticeable contamination and a useful magnetic recording medium abounding in surface smoothness and excelling in the property of buoyancy.